This invention relates to a method and apparatus for correcting a delicate wiring pattern of a highly integrated device.
The pattern width and wiring width have become smaller and smaller recently in devices such as a semiconductor integrated circuit, a GaAs device, a magnetic bubble memory, a Josephson device, and the like. Devices which are from 3.mu. to 2.mu. wide have been realized, and IC devices having wiring widths of 1.5.mu., 1.mu. and sub-microns are now under development. It has been a customary practice to cut the wiring of these IC devices for debugging at the device developing stage, and to cut and connect part of the devices at the fabrication stage of the devices for the relief of defective portion(s), read, adjustment of resistance and capacitance, and so forth.
Cutting of an Al wiring for debugging will now be described as a typical example, but the following can be obviously applied to the wirings made of Au, polysilicon, and the like, too.
In the design and developing process of semiconductor integrated circuits (hereinafter referred to as "ICs"), chips that are produced on a trial basis do not mostly operate as such due to the design and/or process defects. To determine the defective portion(s) of the device, it is necessary in this case to cut a peripheral wiring and to carry out an operation test. However, a passivation film is disposed from time to time on the wiring depending upon the kind of IC devices. If a pattern is greater than 5.mu., the wiring is removed by scratching it off by a thin metal needle fitted to a manipulator, while the device is being observed by a microscope. However, since this method requires a high skill but provides only a low success ratio, it can not be used for IC devices whose pattern is 3.mu. or below.
FIG. 1 of the accompanying drawings illustrates an apparatus for cutting the wiring of an IC device by means of a laser, as disclosed in U.S. Pat. No. 4,190,759, for example. A laser beam 1a emitted from a laaser oscillator 1 is reflected by a mirror 2, and the beam diameter is expanded by a beam expander 3 consisting of a combination of lenses 3a and 3b. A reduction projection image of a rectangular pattern whose size is controlled by variable slits 5 and 6 is formed by a lens 7 on a specimen 8 placed on a table 9. In this case, the light from a lamp 2a for the reference light is reflected by a concave mirror 2b, and is then turned into parallel beams by a lens 2c and the image of the beams is formed on the wiring after passing through the same route as the laser beam. Accordingly, locating of the portion to be removed by the laser, and the like, becomes possible.
In other words, in FIG. 3(a), the Al wiring portion 11 is adjacent to the portion 10 devoid of the wiring. Its section is such as shown in FIG. 2 if a passivation film is not formed. An Al wiring 15 is formed on an Si substrate 13 via an SiO.sub.2 insulating layer 14. In FIG. 1, the image 12 of the slits 5, 6 by the reference light 2d is projected, and the width of the slits is adjusted using micrometers 5, 6 to the width of the wiring portion 11 to be cut. When the laser light is then radiated, the laser light forms the image at exactly the same position 12, and this portion is removed. However, the following problem occurs in this case.
The portion fused by the laser scatters to the peripheral portion and is deposited to the latter, reducing the characteristics of the IC device and causing a short-circuit. Although the SiO.sub.2 insulating layer 14 below the Al wiring 15 is transparent to the laser light, the Si substrate 13 absorbs it. Moreover, the absorbancy of the laser light by the Si is greater than that of the Al. When the Al wiring is to be cut by the laser, the Si substrate below the Al wiring is damaged as shown in FIG. 3(c), and since part of Si is fused and swells, it causes a short-circuit with the Al wiring. The laser also affects adversely the portion which is devoid of the Al wiring and is adjacent thereto, sideways of the Al wiring, causing damage of the Si below the Al wiring. In addition, a short-circuit between the Al and Si is likely to occur in the same way as described above. This is mainly because locating the laser radiation range is difficult, and part of the laser light is radiated to the portion 10 devoid of the Al wiring, and since the insulating layer 14 is transparent to the laser light, it damages a p-type Si portion having conductivity, for example, which is adjacent to the Al wiring, due to thermal conduction and scattering of the Al.
Particularly when a passivation film 18 is coated on the Al wiring 15 as shown in FIG. 4, the Al wiring 15 below the passivation film 18 absorbs the laser light, receives the thermal energy and breaks and projects out of the passivation film because the passivation film 18 is generally made of SiO.sub.2, Si.sub.2 N.sub.4 or the like, is transparent to the laser light and is not therefore worked directly by the laser. For this reason, the scattering Al particles have by far higher energy than when the passiviation film is not provided, are likely to damage the lower and peripheral portions as depicted in FIG. 3(c) and cause cracks on the passivation film itself. Furthermore, the cutting portion involves the possibility that a hole 20 is bored on the passivation film to cause a short-circuit. Since there is no effective method which can easily bury the hole, the device characteristics are likely to drop.
A more fundamental problem is that the method of cutting the wiring by the laser work can not cope with the technical trend that miniaturization and higher density integration of IC devices are contemplated further and the wiring width becomes narrower and narrower from 2.mu. to 1.5.mu. and 1.mu., and further down to the sub-micron order. In other words, it is difficult to obtain a wavelength (0.5.mu. in terms of visible light) spot diameter due to the diffraction limit of the laser light in either of the cases where the image formation projection method shown in FIG. 1 is used and where the beam is thinly contracted by the lens 21 and the specimen is positioned at the focal point, as shown in FIG. 5. Furthermore, the laser work process must pass through the process in which the material first absorbs the laser light and after it is changed to the heat, it scatters the material. Accordingly, influences upon the peripheral portions are unavoidable due to thermal conduction, fusing and jetting, and the like. Thus, the work zone and the zone affected by the heat become inevitably greater than the spot diameter. In other words, the minimum work size of the practical level is about 1.mu. and for this reason, it has been difficult to apply the laser work method of a wiring pattern of below 1.mu..